Force-measuring devices comprising a preload device for mounting a force sensor have been known for quite some time and in different variations. Force-measuring devices of this type can measure applied dynamic and quasi-static forces in at least one direction in space. The force sensors used preferably consists of piezoelectric force sensors, usually so-called piezoelectric measuring washers. Known force measuring devices are used in various applications for measuring different forces, for example in process monitoring.
U.S. Pat. No. 5,824,917, which is hereby incorporated herein by this reference for all purposes, discloses a force-measuring device with a multipart preload device for a force sensor in the form of a double-wedge force-measuring device. This device is suitable for measuring forces between machine parts extending parallel to one another, preferably in accordance with the force bypass principle. The orthogonal distance between the parallel contact surfaces is variable. Two wedges, which can be displaced relative to one another, are connected to one another by means of an adjusting device. The wedges and the force sensor are separably mounted in a pocket-shaped recess between the machine parts, wherein the force sensor and the wedges form a structural unit and the force sensor is integrated into the first wedge. A first measuring surface of the force sensor protrudes from the first wedge and forms first contact surfaces. The force sensor is supported in the first wedge with the second measuring surface. This multipart design should have a small structural height and therefore be easy to install.
The preload device known from U.S. Pat. No. 7,500,398, which is hereby incorporated herein by this reference for all purposes, which forms a force-measuring device together with a force sensor, can be inserted into recesses with parallel walls, as well as into cylindrically symmetrical recesses, and fixed therein under preload such that a more flexible force measurement can be achieved. For example, forces acting upon a roller can thereby be measured by arranging the preload device with the force sensor functionally connected thereto in a cylindrical bore in the roller. The preload device has a multipart design and features a bolt and a pedestal with at least one wedge surface. At least one clamping element is movably arranged on the wedge surface and drawn against a retaining plate by means of a tensioning screw. In a special embodiment, spring elements are used for ensuring that a counterpressure is exerted upon the clamping element by the retaining plate. The force sensor with this compactly designed preload device is separably mounted in a recess under preload by simply tightening the tensioning screw with a suitable tool, wherein the height of the preload device in a preloading direction can be adapted to the size of the recess by tightening the tensioning screw accordingly. Force introduction surfaces of the preload device adapt themselves to the walls of the respective recess such that forces are transmitted to the preload device and to the force sensor. The preload device can be mounted and removed by being accessed from a direction extending transverse to the preloading direction only such that a preload device of this type can be arranged and used in a cylindrical bore in a roller.
The respective preload devices and force-measuring devices comprising a preload device with installed force sensor, which are known from the prior art, have a multipart design and need to be elaborately mounted between the walls of differently shaped recesses under preload by means of a screw-driving tool. The preloaded state of the preload device is achieved by screwing together a wedge-like construction and thereby adjusting the height of the preload device. During this height adjustment, a tensioning screw is tightened until surfaces of the preload device sufficiently press against the recess walls and the force sensor is thereby clamped in position under sufficient preload. In known preload devices, the force sensor is clamped in position asymmetrically, for example, by means of the preload device on one side and by means of a bolt relative to the recess wall on the opposite side. Frictional forces act upon the force sensor during the respective displacement or adjustment of the wedges by means of the tensioning screw.